The present invention relates to continuously transmitting and receiving radar, for example a continuous wave (CW) radar, and more particularly to the suppression of leakage (otherwise known as feedthrough) in such a radar.
A leakage or feedthrough signal comprises an unpropagated portion of the transmitted energy which is fed directly to the radar receiver resulting in saturation of, damage to and/or degradation of sensitivity of the receiver.
The Radar Handbook (Editor M. I. Skolnik, McGraw-Hill, New York, 1970), pages 16-18 and 16-19 discusses minimisation of feedthrough using a dynamic canceller. All dynamic cancellers depend on synthesising a proper amplitude and phase of a signal taken from the transmitter and using this to cancel the feedthrough signal. These pages also mention that microwave feedthrough cancellation is of principal value in preventing saturation and in minimising the effects of AM noise. Because of the correlation effect, FM noise produced by feedthrough tends to cancel in the receiver. Near-in AM and FM noise produced by clutter is also beneficially reduced by a feedthrough servo, since, in nulling out the carrier, it automatically removes both sidebands, whatever their origin, as long as the decorrelation interval is short. Clutter signals from long ranges have both AM and FM noise that is essentially decorrelated, and feed-through nulling of these signals may increase their deviation by a factor of 2 or their power by a factor of 4.
U.S. Pat. No. 3,021,521 discloses a feedthrough nulling system for a CW radar in which a feedthrough signal is cancelled by a correcting signal produced within the radar. More particularly the radar comprises a transmitter coupled by a duplexer to an antenna. This antenna is also coupled by the same duplexer to a receiver. The correcting signal is produced by sampling a portion of the transmitter signal using a first directional coupler. The sample is fed to a phase and amplitude control stage in which it is operated on to produce the correcting signal which is equal in magnitude and opposite in phase to the feedthrough signal. This correcting signal is fed to a second directional coupler where it is combined with the antenna return signal and the feedthrough energy in the signal path from the duplexer to the receiver cancelling the feedthrough signal.
This known technique is primarily for cancelling a feedthrough signal consisting of the carrier frequency. However, U.S. Pat. No. 3,021,521 does not appear to address itself to overcoming the problems of (1) FM noise sidebands present in the leakage signal, (2) the leakage signal power being high enough to degrade the sensitivity of or damage the receiver, and (3) the AM sidebands of the leakage being high enough to degrade the sensitivity of the receiver. Other aspects of the method/technique for suppressing leakage should take into account that the amplitude of the correcting signal is limited by what can be handled by a modulator and that if a control loop is provided having a broad band then there is a danger of the wanted signals being cancelled.
A method of solving the first of the above-mentioned problems is disclosed in British Patent Specification No. 2147473B which describes a method of FM noise reduction in a CW radar system. The system comprises a master oscillator, means for transmitting an RF signal derived from the oscillator and for receiving a return signal, and a mixer having a local-oscillator port and a signal port. A local oscillator signal is obtained by coupling-out a portion of the master oscillator signal. A leakage signal may reach the signal port of the mixer otherwise than along the propagation path of the local oscillator signal and without being reflected externally of the system, for example by reflection of the signal supplied to the transmitting means. In order to minimise noise in the mixer due to FM noise in the leakage signal, the electrical lengths over the operating frequency range of the system from the master oscillator to the local oscillator port and to the signal port of the mixer of the propagation paths of the local oscillator signal and the leakage signal respectively are made substantially equal, for example by including a delay line in the local oscillator signal path. Since FM noise power decreases with increasing offset from the carrier frequency (that is the transmitted frequency), this known method of noise reduction is particularly suited to alleviating noise at low intermediate frequencies (IFs). Additionally noise reduction systems are particularly useful with frequencies in the millimeter-wave range for which currently available oscillators tend to be rather noisy.